In automation technology, especially in process automation, however, also in manufacturing automation, field devices are applied, which serve for determining and monitoring process variables. In connection with the invention, the terminology, field devices, refers to both actuators as well as also sensors. Sensors, for example, register different process variables, such as fill level, flow, pH-value, turbidity, substance concentration, pressure, temperature, moisture, conductivity, density and viscosity. A large number of such field devices are available from members of the firm, Endress+Hauser. Such field devices produce signals in the high-frequency region, which need to be evaluated, in order that the process variable can be determined from the evaluation of the high-frequency signal.
According to the state of the art, the frequency of a high-frequency signal is determined by fast counters. For this, the frequency is divided down a number of times, in order that the frequency can be handled by digital ICs. Alternatively or supplementally, also a mixer can be used, in order to remove the frequency offset (in the present case the 2.4 GHz). However, these concepts face certain limits at high frequencies. The general problem in the case of frequency dividers is that the actual wanted frequency is likewise divided by the same factor, which means that one must measure more exactly. Also, “normal” digital circuits reach, most often, only into frequency ranges of some 100 MHz. Chips for higher frequencies are, most often, expensive and/or possess an increased electrical current draw. Mixers are likewise expensive and have, most often, a high electrical current draw, i.e. a high conversion loss. Furthermore, a mixer must, in turn, be fed a high-frequency reference-signal having a very exactly maintained frequency.